They aren't guessing heads or tails, they give odds for each event. It's more like eyeballing a thousand coins to guess how fair they are, and then flipping each one just once.

Some are weighted to be 99% heads, others are 10% heads etc.

You could have 1,000,000 people guess random percentages for each coin, but suppose 10 of the coins are weighted 100% heads. To guess within 25% of the true value for all 10 of those coins would be roughly 1 in a million.

So a lucky guy guesses within 25% for all 10, he'd have another 990 coins he's being judged on.